The manufacture of a ceramic matrix composite (CMC) part typically includes laying up pre-impregnated composite fibers having a matrix material already present (prepreg) to form the geometry of the part (pre-form), autoclaving and burning out the pre-form, infiltrating the burned-out pre-form with the melting matrix material, and any machining or further treatments of the pre-form. Infiltrating the pre-form may include depositing the ceramic matrix out of a gas mixture, pyrolyzing a pre-ceramic polymer, chemically reacting elements, sintering, generally in the temperature range of 925 to 1650° C. (1700 to 3000° F.), or electrophoretically depositing a ceramic powder. With respect to turbine airfoils, the CMC may be located over a metal spar to form only the outer surface of the airfoil.
Examples of CMC materials include, but are not limited to, carbon-fiber-reinforced carbon (C/C), carbon-fiber-reinforced silicon carbide (C/SiC), silicon-carbide-fiber-reinforced silicon carbide (SiC/SiC), alumina-fiber-reinforced alumina (Al2O3/Al2O3), or combinations thereof. The CMC may have increased elongation, fracture toughness, thermal shock, dynamic load capability, and anisotropic properties as compared to a monolithic ceramic structure.
Conventional CMC blades typically only include one dovetail, which has two opposing pressure faces that contact the rotor tangs. As a result, the area that is required on each pressure face is high, and the fillet from the airfoil that transitions to these pressure faces may be large. If the fillet and pressure faces are large enough, the reduction in total rotor circumferential tang length may be reduced to a point at which the rotor is compromised. Additionally, it would be preferable for the fillet and neck region of the composite blade to be larger in order to maintain safe operation and reduced interlaminar tension generally seen in the neck region. CMC blades are highly orthotropic, and bending from the dovetail pressure contact faces induces a moment that attempts to pry the plies apart in the neck region perpendicular to the radial loading direction.
A lower flank angle on the CMC dovetail increases fillet and interlaminar tension (ILT) stresses and increases wear concerns due to a higher normal force, but there is a risk of lock up for higher flank angles.